Rotational screws for mixing in the prior art have been introduced; particularly that of barrier type is disclosed in U.S. Pat. No. 3486192 (See FIG. 6); and that of damflight type is disclosed in Japanese Unexamined Patent Publication No. 11505/1967 (see FIG. 7).
The barrier type screw 60, of the former, has a mixing portion 62 thereof which is equal to flight portions thereof in diameter and provided in the central region of a screw shaft 61. On the periphery of the mixing portion 62 there are a plurality of axially extending mixing recesses 63. In operation with this barrier type screw 60, while material supplied into the mixing recesses 63 is transported in the axial direction as the screw shaft 61 rotates, a portion of the material moves in a direction opposite to the rotating direction of the screw shaft 61 and will flow into the nearest recess 63 situated in a circumferential direction. This causes the material to flow disorderly in two directions on the mixing portion 62 and thus, to be prevented from uniformly mixing with additives.
The dam-flight type screw 70, of the latter, has a lead of main flight 72 disposed on a screw shaft 71 thereof and also, a lead of auxiliary flight 73 having a large distance or pitch. In addition, a mixing portion 74 of the screw 70 is formed such that the distance between the flights 72 and 73 becomes narrow in the transporting direction. Thereby, as the screw shaft 71 rotates, a portion of material moves across the auxiliary flight 73 and will be joined with the flow of material moving along the back side of the auxiliary flight 73. However, this action does not result in continuous and appropriate mixing of the material, but only in agitation of the same with both of the flights 72 and 73. It will then be difficult to provide a uniform mixture of material constantly by intimately mixing the material with additives in the arrangement.